There has been known a printer system in which a host computer such as a personal computer (PC) or a work station (WS) and a printer such as a laser printer are connected to each other so that intercommunication can be made. In such printer system, when an image is to be output to a printer or a display, pixel data (such as RGB as one of signals to be sent to a color display, or CMYK for color printing) for each color as original data for the image is scanned on the printer or the host computer, and the data is converted to a VRAM (video memory) format line by line. When the image on the basis of the data (drawing data) that has been converted to the VRAM format is to be output by the printer, the drawing data is converted to a printer language to be output to the printer as print data together with a control signal.
When the density level of each color is not 0 (%) or 255 (%), the pixel data is subjected to masking by a pattern corresponding to a color level in the process of converting the data to the video memory format. As the processing of masking, a dither pattern (matrix) represented by a block consisting of 4×4 pixels or 6×6 pixels or so at each color level is used. The masking using the dither pattern is referred to as a dither method. The dither method is a technique for adjusting positions and arrangement of pixels for different colors, and artificially reproducing an intermediate color. According to the dither method, gradations can be reproduced with a smaller number of colors by finely changing a color distribution using two types of colors such as a dark color and a light color.
The dither data used for the dither method is generally designed so as to match an object for drawing (drawing object) such as a photograph and a character. The drawing data created for drawing each object is broadly classified to three types: character data for drawing characters, photograph data for drawing photographs, and graphics data for drawing graphics. The dither data used for drawing each of the data includes data designed for the character data so as to improve reproductivity of thin lines (dither data for characters) and data designed for the photograph data so as to reproduce changes in color densities as naturally as possible.
Conventionally, when each image of the three types of drawing object is drawn using the dither data for characters or the dither data for photographs, one of the dither data is selected. The correlation between the dither data and the drawing objects is as shown in the table 1 explained below.
[Table 1]
                1 Drawing object        2 Character        3 Photograph        4 Graphics        5 Dither data to be used        6 Dither data for characters        7 Dither data for photographs        
However, the conventional method for selecting the dither data as shown above has some problems explained below. A character as drawing data includes one to be sent as an image, other than ordinary character data, like data obtained by rotating a character used in PageMaker (that is typical DTP software provided by Adobe Systems of USA) or the like. In such image data, although the drawing object is a character, it is generally drawn using dither data for photographs. Therefore, if the character is a fine one, it is sometimes hard to be found out. Further, in regard to data for gradation frequently used in PowerPoint (that is presentation software provided by Microsoft Corporation), the data is drawn based on the dither data for characters, therefore, reproductivity of gradation may often be degraded.
In the conventional printer system, when drawing data received from the application is to be sent a printer (or a device), only the data obtained by converting the drawing data to a page description language and a control code are sent to the printer (or the device). Therefore, it is hard for the printer controller to identify the type of drawing data from the data and control code. Hence, dither data matching the type of drawing object is not always selected, thus the dither method optimal to the drawing object may not be executed, which causes degradation in image quality to occur.
A graphic created based on graphics data includes one with a portion of area fill surrounded by a line drawing (area fill) and one with no area fill. In the conventional printer system, the graphics data is subjected to drawing processing using the dither data for characters regardless of presence or absence of area fill in the graphics as shown in table 1.
In the image formed on the basis of the graphics data with area fill, there occurs such inconvenience that reproductivity of gradations in the portion of area fill is degraded. Further, when all the graphics data is subjected to drawing processing using the dither data for photographs in order to resolve the inconvenience, the degree of reproductivity of comparatively thinner lines may be decreased.
In fact, the dither data takes time to design and also requires comparatively larger storage capacity for its storage in the printer system. Therefore, by using respective dither data dedicated to the graphics data with area fill and the graphics data with no area fill, development of the printer system is restricted. This also causes the memory capacity provided in the printer system to be upsized.
Further, the printer system sometimes requires to convert pixel data created by a CAD (Computer Aided Design) application to print data and draw an image based on the print data. The CAD application can create the pixel data for a thin line of a dot 1 pixel wide. Therefore, when an image is drawn based on the pixel data created by the CAD application, the printer system is also required to form an image with higher quality.